1. Field of the Invention
The present invention relates to a beam position adjusting method for an optical scanning device used in, e.g. an electrophotographic digital copying machine.
2. Description of the Related Art
In the technical field of image forming apparatuses such as high-speed digital copying machines and high-speed printers, there is known an optical scanning device using, e.g. a multi-beam exposure device which simultaneously scans two or more beams and exposes a charged area on a photosensitive drum.
The multi-beam exposure device comprises, for example, a plurality of semiconductor laser elements for emitting laser beams; an optical member for adjusting the cross-sectional beam dimensions of the laser beams emitted from the semiconductor laser elements in accordance with a required resolution and setting an inter-beam distance between the laser beams in a sub-scan direction (a direction perpendicular to the axis of the photosensitive drum); a deflection device for deflecting and scanning the laser beams together in a main scan direction (an axial direction of the photosensitive drum); and a focusing lens system for guiding and focusing the laser beams deflected by the deflection device on a charged area on the photosensitive drum.
A galvano-mirror is used as an optical member used in the multi-beam exposure device. The galvano-mirror can minutely alter the angle of the mirror surface by controlling current supplied to an electromagnetic coil.
For example, in a multi-beam exposure device that simultaneously scans two laser beams, that is, a stationary laser beam and a movable laser beam, the angle of the mirror surface of the galvano-mirror is finely adjusted to control the distance (or the position) of the movable laser beam in the sub-scan direction.
The adjustment of the distance is effected by using a sensor such as a photodiode. Specifically, a laser beam is caused to scan the light-receiving surface of the sensor and a position on the light-receiving surface, at which the laser beam passes, is detected. This operation is performed for each laser beam. Thus, the distance between the laser beams is detected, and is adjusted based on the detection result.
The position of the stationary laser beam in the sub-scan direction is fixed and, theoretically, it does not vary. However, due to a variation in temperature, voltage or the like, the position may vary. In such a case, an inter-beam distance between the stationary laser beam and movable laser beam may deviate from a set value, leading to such a problem that precise beam scanning cannot be performed.